Yayoi-era polished stone semi-lunar reaping knives, used to cut ears of rice before the apparition of the sickle.
Those kind of stones knives could be found in many cultures around the world. As stated by archeologist OP, what is super interesting is that they are still used in some areas (last pictures is captured from in Chinese video and shows a similar tool made from iron).
Tbh, they look so handy I'd love to own one for foraging during strolls!















